There is known an electrolytically chromated steel sheet having on at least one surface of a steel sheet a chromate film comprising a metallic chromium layer as a lower layer and a hydrated chromium oxide layer as an upper layer formed on the metallic chromium layer. The metallic chromium layer as the lower layer usually has a thickness of from about 0.005 to about 0.03 .mu.m, and the hydrated chromium oxide layer as the upper layer usually has a thickness of from about 0.01 to about 0.04 .mu.m.
Methods for manufacturing the above-mentioned electrolytically chromated steel sheet are broadly divided into the following two classes:
(1) One-step method:
This method comprises subjecting a steel sheet to a cathodic electrolytic chromate treatment in an acidic electrolytic chromating solution comprising at least one of chromic anhydride, chromate and bichromate as a main agent, and at least one of sulfuric acid, sulfate and fluorine compound as an assistant agent, to form on at least one surface of the steel sheet simultaneously a metallic chromium layer as a lower layer and a hydrated chromium oxide layer as an upper layer.
(2) Two-step method:
This method comprises subjecting a steel sheet to a first cathodic electrolytic chromate treatment in an acidic electrolytic chromating solution comprising at least one of chromic anhydride, chromate and bichromate as a main agent, and at least one of sulfuric acid, sulfate and fluorine compound as an assistant agent, to form on at least one surface of the steel sheet simultaneously a metallic chromium layer as a lower layer and a hydrated chromium oxide layer as an upper layer (a first step); and then, after removing the thus formed hydrated chromium oxide layer through dissolution, subjecting the steel sheet from which the hydrated chromium oxide layer has been removed to a second cathodic electrolytic chromate treatment in another acidic electrolytic chromating solution comprising at least one of chromic anhydride, chromate and bichromate as a main agent, to form again a new hydrated chromium oxide layer as an upper layer on the metallic chromium layer as the lower layer (a second step).
The electrolytically chromated steel sheet manufactured as described above is excellent not only in a corrosion resistance but also in a paint adhesion between the chromating film and a paint film formed thereon, i.e., a primary paint adhesion, and is less expensive as compared with a tin-plated steel sheet. The electrolytically chromated steel sheet is therefore widely used in place of the tin-plated steel sheet as a material for cans such as a food can, a pail can, an 18-l can and an oil can. A soldered can made of the tin-plated steel sheet, which comprises an upper lid, a bottom lid and a drum of which the seam is soldered, has been used as a can for a soft drink. In replacement of the soldered can, recently, a cemented can made of the electrolytically chromated steel sheet, which comprises an upper lid, a bottom lid, and a drum of which the seam is cemented with a nylon adhesive, has come to be employed. The cemented can made of the electrolytically chromated steel sheet has become popular for the following reasons: The cemented can made of the electrolytically chromated steel sheet is less expensive than the soldered can made of the tin-plated steel sheet. In addition, when the cemented can is filled with a carbonated drink, for example, the carbonated drink never leaks from the seam and the degree of vacuum in the can never decreases because of the excellent primary paint adhesion of the electrolytically chromated steel sheet.
A cemented can is usually manufactured by a process comprising: forming a paint film on each of the chromating films on the both surfaces of a electrolytically chromated steel sheet having prescribed dimensions, then forming the electrolytically chromated steel sheet having the paint films thereon into a drum of can, cementing the seam of the overlapping portions of the drum of can with an adhesive, and then, securing an upper lid and a bottom lid to the drum with the thus cemented seam.
A high-temperature content such as a fruit juice heated to a temperature of from 90.degree. to 100.degree. C. for sterilization may be charged into the thus manufactured cemented can made of the electrolytically chromated steel sheet, or the above-mentioned cemented can filled with a content may be heated by means of pressurized steam at a temperature of about 130.degree. C. for sterilization of the content. However, when filling the cemented can made of the electrolytically chromated steel sheet with the high-temperature content, or when heating the cemented can filled with the content by means of high-temperature steam, paint adhesion between the chromating film and the paint film formed thereon, i.e., secondary paint adhesion in high-temperature and high-humidity environment decreases.
As a result, the seam of the can suffering from the most serious stress is broken, and the content of the can leaks out through the broken portion of the seam, or the degree of vacuum in the can is reduced. This deterioration of the secondary paint adhesion is attributable to the fact that water penetrates between the chromating film on the seam portion of the drum and the paint film formed thereon and reduces adhesion between these films. A higher penetrating rate of water therefore leads to more serious deterioration of the secondary paint adhesion. The electrolytically chromated steel sheet is usually manufactured, as described above, by the application of any of the one-step method and the two-step method. None of these methods can prevent deterioration of the secondary paint adhesion.
The electrolytically chromated steel sheet is used also as a material for a two-piece can comprising a cup-shaped can body and an upper lid, in addition to the application mentioned above for a cemented can. However, the electrolytically chromated steel sheet is not used so popularly as a material for a welded can comprising an upper lid, a lower lid and a drum having a seam welded by an electric resistance welding, because of a low weldability of the electrolytically chromated steel sheet. However, demand for the welded can is increasing because of the high strength of the seam thereof. For the purpose of using the electrolytically chromated steel sheet as a material for the welded can, therefore, improvement of weldability thereof is now demanded.
The electrolytically chromated steel sheet has a low weldability for the following reasons: Both the metallic chromium layer as the lower layer and the hydrated chromium oxide layer as the upper layer, which form the chromating film, are not thermally conductive, and furthermore, the hydrated chromium oxide layer as the upper layer is not electrically conductive. Therefore, when welding the seam of the overlapping portions of the cylinder of the can by electric resistance welding, the hydrated chromium oxide layer as the upper layer becomes an electrically insulating layer, thus increasing the value of contact resistance at the portion to be welded. The value of contact resistance serves as a criterion for determining whether excessive electric current locally flows or not during welding. More specifically, when the value of contact resistance is high, excessive electricity tends to locally flow because of the narrow path for welding electric current. The electrolytically chromated steel sheet has a value of contact resistance within the range of from 10.sup.2 to 10.sup.5 .mu..OMEGA./mm.sup.2, which is far higher than that of the other surface-treated steel sheets for the welded can. Therefore, when welding the electrolytically chromated steel sheet by the electric resistance welding, the value of welding current is low immediately after the start of welding, and after the lapse of a certain period of time, reaches a prescribed value of welding current. As a result, the electrolytically chromated steel sheet locally generates heat at the beginning of welding to produce a splash, and defects such as blowholes are produced at the welded joint. When welding the electrolytically chromated steel sheet, therefore, it has conventionally been necessary to remove the chromating film at the portion to be welded through grinding, for example, which has required much time and labor.
As a means to solve the above-mentioned problems of the electrolytically chromated steel sheet, i.e., to prevent deterioration of the secondary paint adhesion and the weldability, a known method comprises forming numerous granular projections over the entire surface of the metallic chromium layer as the lower layer of the chromating film. The electrolytically chromated steel sheet having the chromating film which includes the metallic chromium layer as a lower layer provided with numerous granular projections over the entire surface thereof, has the following characteristics:
(1) When the above-mentioned electrolytically chromated steel sheet is used as a material for a cemented can in which a seam of the drum is cemented with an adhesive, penetration of water between the chromating film and the paint film formed thereon is prevented. The secondary paint adhesion is accordingly improved.
(2) When the above-mentioned electrolytically chromated steel sheet is used as a material for a welded can in which a seam of the drum is welded by electric resistance welding, the hydrated chromium oxide layer as the upper layer, which is not electrically conductive, is broken during the electric resistance welding by the numerous granular projections formed on the entire surface of the metallic chromium layer as the lower layer, thus reducing the value of contact resistance of the portion to be welded and improving weldability.
For the purpose of forming the numerous granular projections over the entire surface of the metallic chromium layer as the lower layer of the chromating film, the following methods are known:
(1) A method for manufacturing an electrolytically chromated steel sheet, disclosed in Japanese patent provisional publication No. 62-54,096 dated Mar. 9, 19867, which comprises: subjecting a steel sheet to an anodic electrolytic treatment at least once in the middle of a plurality of runs of application of a cathodic electrolytic chromate treatment to the steel sheet so as to form numerous granular projections on the entire surface of the metallic chromium layer of the chromating film (hereinafter referred to as the "Prior Art 1").
(2) A paper under the title of "the effect of crystallographic orientation on the growth of electrodeposited metallic chromium", appearing in the "Metal Surface Technology", a journal of the Metal Finishing Society of Japan, Vol. 35, No. 7, pages 34-38, issued on July 1, 1984, which reveals the fact that, when a steel sheet is subjected to a plurality of runs of cathodic electrolytic chromate treatment intermittently in an acidic electrolytic chromating solution, numerous granular projections are formed over the entire surface of the metallic chromium layer of the chromating film formed on at least one surface of the steel sheet (hereinafter referred to as the "Prior Art 2").
The above-mentioned Prior Art 1 has the following problems:
(1) When the steel sheet is subjected to the anodic electrolytic treatment in the middle of a plurality of runs of application of the cathodic electrolytic chromate treatment to the steel sheet, numerous granular projections are formed over the entire surface of the metallic chromium layer of the chromating film, but the thus formed granular projections have a very small average particle size of up to about 0.05 .mu.m. As a result, reflected light causes diffraction and interference in the metallic chromium layer. This makes the surface of the electrolytically chromated steel sheet look black or brown, thus seriously impairing the surface hue.
(2) Production of hydrogen gas upon precipitation of metallic chromium usually results in a low precipitation efficiency of about 20% of metallic chromium in the cathodic electrolytic chromate treatment. From the point of view of the consumption of electricity required for the cathodic electrolytic chromate treatment and productivity of the process, therefore, there is a demand for improvement of precipitation efficiency of metallic chromium. However, if the steel sheet is subjected to the anodic electrolytic treatment in the middle of a plurality of runs of application of the cathodic electrolytic chromate treatment to the steel sheet, part of the metallic chromium layer thus formed is dissolved by the anodic electrolytic treatment, thus seriously reducing the precipitation efficiency of metallic chromium.
The above-mentioned Prior Art 2 has the following problems: in order to form numerous granular projections over the entire surface of the metallic chromium layer of the chromating film formed on at least one surface of the steel sheet through intermittent application of the plurality of runs of the cathodic electrolytic chromate treatment, it is necessary to provide a long non-energizing period of time between the plurality of runs of the cathodic electrolytic chromate treatment, or to use an extremely low travelling speed of the steel sheet for the plurality of runs of the cathodic electrolytic chromate treatment. As a result, it is necessary to provide large-scale manufacturing facilities of the electrolytically chromated steel sheet, or the manufacturing efficiency is largely reduced.
Under such circumstances, there is a strong demand for development of a method for efficiently manufacturing an electrolytically chromated steel sheet excellent in secondary paint adhesion and weldability and having a satisfactory surface hue, but such a method has not as yet been proposed.